This invention relates to crossed-field amplifier tubes for the amplification of extremely high microwave frequencies (EHF) over a broad band. The prior capability of generating power at EHF is limited by a number of factors. Power can be obtained over narrow bandwidths (1-2 GHz) by the use of coupled-cavity tubes (1-2 KW). Helix traveling wave tubes are presently under development to attempt to provide power output in the frequency ranges 18.0-26.5 and 26.5-40.0 GHz range. Other limitations on the type of tubes suitable for generating power in the above frequency ranges are the physical size, weight, and power supply requirements for these tubes. A recognized need in the microwave amplifier field is for amplifier tubes capable of efficient, broadband performance at high power levels at millimeter wavelengths. The problem has been that microwave tubes become smaller at millimeter wavelengths and are thus limited in power. It is therefore an object of this invention to provide a new type of high power microwave amplifier which provides high power amplification over a broad frequency range at EHF frequencies.